The measurement of the gas flow in a pipeline typically requires a series of meters located along the pipeline, each meter recording data that is sufficient to determine the volume of gas flowing through the meter. The two most commonly used techniques for measuring gas volume are orifice measurements and PVT measurements. Orifice measurement meters are typically found on large flow stations, and operate by recording pressure and the pressure drop across an orifice placed in the pipeline. These two quantities are plotted as a function of time on a circular chart. Gas temperature is also recorded, on the same chart or on a separate chart, and the gas temperature data is combined with measurements of the areas under the pressure and pressure drop curves to calculate flow volume. PVT measurement is different from the orifice measurement described above, in that a direct volume reading is recorded on the chart, along with the pressure and temperature. These three values are sufficient to calculate corrected volume figures.
The problem with PVT charts and orifice charts having temperature readings is that three readings must be made simultaneously to make accurate calculations. Thus, many companies drop temperature off of the orifice chart to simplify the calculation. The separate temperature chart is usually sight read or estimated.
Charts vary in length from 24 hours (high volume measurement stations) to up to 31 days in low volume stations. Seven day charts are the most common multi-day charts. Volume calculations are often broken down into daily volumes from these charts. Charts come in from the field from a variety of meter types, and in several time lengths. After being checked in and logged as to customer, meter number and chart date, the charts are sorted and either sight read, integrated by a special machine that calculates the total area under one or two plotted pen lines, or optically scanned. When integration is used, orifice charts that have both pressure and differential plots must have both curves continuously monitored through the entire chart interval to calculate volume. A machine that has two pens, controlled by hand levers, and a rotating table, controlled by a speed pedal, is used by a skilled operator to exactly reproduce the meter movements and subsequently calculate average pressure over the whole chart, and a quantity known as extension that is derived from pressure and differential pressure. Typically, 2,000 readings are taken on a full chart. Once this step is completed, the resulting values are printed on the back of the chart for entry into the volume calculation computer. Prior scanning devices have been used to measure the pressure and differential curves from orifice meters. However, in the past, temperature could not be recorded on such charts, since no scanners were capable of accepting charts having more than two curves.
In the past, optical scanning has invariably required a preparatory step in which an operator cleans up the chart to be acceptable input for the scanner. The clean-up of a chart to be scanned basically requires marking or erasing areas of the chart to produce acceptable scanner input. The clean up of charts for scanners can be a time consuming process, involving the use of a white-out material such as used for typewriters, white tape, or felt tip pens to accentuate dim scans or extend data where it was missing due to a slow clock. The individuals doing chart clean up also must have a high level of training.
With prior scanners, the chart color typically must be pale blue, a color difficult to sight read, and specially manufactured red and black inks are also required. In addition, when two curves cross on a given chart, conventional optical scanners have no means to track which curve is which after they diverge. A result of this limitation is that on a two-graph chart, pressure must either be the inner or outer curve, and charts must be sorted depending on the relative positions of the two curves. If two curves do cross, the pressure figure can typically not be determined. Recently, however, some scanners have become available that can distinguish up to two different colors from one another. However, temperature data can still not be taken with pressure data, and temperature data has therefore been sight averaged for systems employing such scanners.